In a wide variety of manufacturing and steel processing applications, it is often desirable or necessary to join together sheets or strips of steel or similar materials such as alloys or the like, such as by welding. This also may include joining sheets of different material or thickness in order to custom make or "tailor" a part. Such joining can be accomplished by conventional seam welding equipment, arc welding apparatus, high energy lasers, electron beam or plasma arc welding devices.
Because the quality of the seam weld must be at least equivalent to the base metal in mechanical and microstructural properties, it is essential to optimize the quality of the weld in many of today's advanced technology applications. In particular, the quality of the weld can affect the overall microstructure, microhardness, tensile properties, formability, fatigue strength and fracture toughness, which all directly affect the overall value of the joining process and the character of the resulting joined pieces. In turn, the ability of the welding device to accurately track the gap between two abutted sheets to be joined is critical to ensuring an optimal weld, especially in high speed welding applications utilizing tightly focused energy beams where the application point of the weld must be continuously maintained in close alignment with the center of the gap.
A uniform weld profile, in large part, is determined not only by the registration of the opposing proximal edges of workpieces to be joined, but by the ability of a tracking system to compensate for mis-positioning of the gap between two abutted workpieces in a translational sense. Accordingly, there have been substantial efforts to develop practical and reliable seam tracking systems for use in such applications.
One approach for the tracking of the gap to be welded is the use of contacting probes. Such systems generally utilize the physical characteristics of the workpieces and/or their contiguous gap as a mechanical guide for a sliding or rolling mechanism to which the welding head is linked. A tracking system of this design precedes, or is laterally spaced to the side of, the welding head and occupies a significant physical volume adjacent the vicinity of the point of welding (or welding zone). Implementation of contacting systems can vary from simple mechanical designs in which forward motion by the workpieces produces an aligning transverse force on the welding head manipulator, to sophisticated, computer controlled electro-mechanical systems in which forces on the probe are sensed electronically and utilized to activate drive motors. These systems, however, often lose contact with the joint, which can interrupt the operation of the tracking device and compromise the weld quality. Generally, such systems are limited to low welding speeds and particular applications (e.g., where there are predetermined or controlled physical characteristics of the workpieces to be joined and their contiguous gap), and are subject to fluctuations in sensitivity as well as damage from heat, wear and other abuse.
Accordingly, use of various non-contacting probes has been attempted to address the aforementioned limitations, while providing more information about the weld joint (especially the edge conditions thereof). Whereas a contacting probe system may sense only a predetermined number (e.g., one or two) of points along a gap, a non-contacting sensor may be able to repeatedly scan across the gap to be welded to provide an effective "map" of the gap geometry. Many non-contacting sensing media have been investigated, including those utilizing magnetics and electro-magnetics (reluctance and eddy current types), fluidics and pneumatics, sound propagation, and visible and infrared imaging. In this area, use of visible imaging and arc sensing have received the most recent serious attention.
Arc sensing, as exemplified by Abshire, et al. U.S. Pat. No. 4,806,732, is based on the relationship that the electrical characteristics of the arc depend on the distance between the welding head and the workpiece. Thus, motion of the welding head back and forth across the gap produces a varying electrical response (arc voltage and/or current) which can be electronically analyzed for recognition of gap details. This is advantageous because there is no sensor of appreciable size to inhibit the welding head, and the sensing is performed at the point of welding. On the other hand, in arc sensing systems, the welding head or arc must be oscillated transverse to the gap or joint, and small electrical variations may be difficult to detect and distinguish from inherently fluctuating arc characteristics. Additionally, the joint cannot be sensed prior to arc initiation for prepositioning of the torch (since electrical characteristics of the arc cannot be sensed until the welding head is between the workpieces), and dimensional resolution of the system is also limited.
Visual imaging systems have also been utilized in various tracking systems. One such system is generally referred to as a non-structured light system, where an imaging device views the weld area (usually the joint in advance of the point of welding) with general illumination, such as that provided by the welding arc or an auxiliary high intensity light source. The image is analyzed according to the varying levels of illumination viewed for features representative of the joint preparation. This type of visual imaging system has been especially successful for the welding of butt joints where a distinct joint clearance exists between abutted flat workpieces. Although such systems are non-contacting, the imaging system is generally directed to an area in advance (or ahead) of the point of welding and may thus be considered intrusive to the weld area and highly directional.
Visual imaging has also been utilized in tracking systems which provide structured light systems having a particular pattern of light projection. In these systems, the light may be a focused beam, or a plane or multiple planes of light projected at an angle to the imaging system. Recognition of the particular light wavelength allows sensing of a reflection pattern of the light from the workpiece, which in turn permits an optical triangulation calculation to be performed to locate a point or points on the workpiece. This allows various ranges of resolution for the joint region contour to be analyzed by the projection and imaging system. The light projection device usually is a high intensity strobe lamp or laser having an intensity or distinct wavelength which can be discerned even in the presence of the arc. The structured light system also generally senses an area spaced ahead of the zone of welding, and may also be considered intrusive to the welding area and directional, although non-contacting. Additionally, such systems generally are complicated by requiring additional computer memory and software to perform algorithms for use with the triangulation techniques.
An example of a structured light system as described above is the Seampilot Optical Profile Sensor System manufactured by Oldelft Corporation of America. Besides employing triangulation techniques, the Seampilot system compares camera readings of the monitored light reflected with programmed templates or predetermined patterns of reflections stored in a computer, whereby corrections are made in the positioning of the welding device to compensate for camera readings which are inconsistent with the stored template patterns. Therefore, the Seampilot system causes adjustment of welding device position only in response to comparisons with predetermined template patterns (sometimes referred to as a closed loop design). As such, in addition to the other deficiencies of this system as described above, the accuracy of this system is limited to the accuracy of the programmed template or templates available for comparison, and adaptation of the system to new applications is inherently cumbersome.
Another system which has been considered in the industry includes a visual system which is coaxial with the welding zone, as described in an article by R. W. Richardson, D. A. Gutow, R. A. Anderson and D. F. Fausen entitled "Coaxial Arc Weld Pool Viewing For Process Monitoring Control." (Welding Journal, March 1984, pp. 43-50). Specifically, the imaging system of this design has been integrated into the welding torch device itself. This configuration is thereby different from an imaging system external to the welding torch, which is generally mounted separately from and oriented at an oblique angle relative to the welding torch axis. However, as with all vision systems of the prior art, the imaging and monitoring capabilities of this coaxial system are impaired by the inability to control smoke, spatter and other welding debris inherently present immediately adjacent the welding zone.